1. Field of the Invention
The present invention relates to a hybrid system for a hybrid vehicle using an engine and a rotary machine as power sources.
2. Description of Related Art
As such a type of hybrid system, there has been known a hybrid system including an engine, two rotary machines, and a power split device (a planet gear mechanism). In the hybrid system, a rotating shaft of the engine, a rotating shaft of a first rotary machine, a rotating shaft of a second rotary machine, and driving wheels are connected to each rotating element of the power split device. For example, in a hybrid system described in Japanese Patent Application Publication No. 2008-120234 (JP 2008-120234 A), a planet gear mechanism operable as a speed change unit is placed between an engine and a power split device. The speed change unit includes: a rotating element connected to a rotating shaft of an engine; and a rotating element connected to a rotating element (except for rotating elements connected to a rotating shaft of a first rotary machine, a rotating shaft of a second rotary machine, and driving wheels) of the power split device. Further, the hybrid system of JP 2008-120234 A is provided with an engaging device (a switching device) configured to switch the speed change unit between a state where a differential rotation is performable and a state where the differential rotation is not performable, so as to change a transmission gear ratio of the speed change unit. Note that hybrid systems described in Japanese Patent Application Publication No. 2008-265598 (JP 2008-265598 A) and Japanese Patent Application Publication No. 2008-265600 (JP 2008-265600 A) are each provided with a clutch (a fixed element) configured to fix a rotating shaft of an engine. In such a hybrid system, when the clutch is engaged to stop a rotation of the rotating shaft of the engine, it is possible to perform traveling by use of both powers from a first rotary machine and a second rotary machine. Accordingly, in this hybrid system, the engine, the first rotary machine, the second rotary machine, and the clutch are controlled according to a requested driving force, and a power source to be used for traveling is selected therefrom. Further, in the hybrid system of JP 2008-265600 A, at the time of traveling by use of both powers from the first rotary machine and the second rotary machine, torque split between the first rotary machine and the second rotary machine is determined according to respective efficiencies of the first rotary machine and the second rotary machine.